1. Field of the Invention
The present invention relates to a PIN photodiode used as a receiving element for converting a light wave to an electric signal in an optical communication, more particularly relates to improvements in characteristics such as a frequency characteristic or a wavelength sensitivity characteristic.
2. Background Art
Examples of conventional PIN photodiodes are shown in FIGS. 4 and 7. A high concentration N-type layer (an N.sup.30 layer) 91 in a PIN photodiode 90 shown in FIG. 4(A) is formed by a diffusion from one surface of a high specific resistance silicon wafer 92A. The thickness of an I-type layer 92 as a remaining part is determined by the control of the diffusion. Further, a high concentration P-type layer (a P.sup.30 layer) 93 is formed by a diffusion from the opposite surface to the N-type layer 91, thereby forming the basic constitution of a PIN photodiode 90.
Since the high concentration N-type layer (an N.sup.+ layer) 91 is formed by the diffusion as described, some N.sup.+ carriers (hereinafter referred to as "N carriers") are diffused into the I-type layer 92. The concentration distribution B1 of the N carriers in the N-type layer 91 and the I-type layer 92 will have a gentle slope as shown in FIG. 4(B). This means a depletion layer 94 is a shallow layer which does not reach a junction surface 95 between the N-type layer 91 and the I-type layer 92.
An I-type layer 82 in a PIN photodiode 80 shown in FIG. 7(A) is formed by an epitaxial growth on one surface of a high concentration N-type wafer 81A as shown in FIG. 8. In this conventional art, the thickness of the I-type layer 82 is determined by, for example, a time control of the epitaxial growth. Further, a P-type layer 83 is formed as well as the above-described conventional art.
Since the I-type layer 82 is formed by the epitaxial growth on the preformed N-type layer (the N.sup.+ layer) 81, it is possible to form the I-type layer 82 which does not contain so many N carriers. The concentration distribution B2 of the N carriers in the N-type layer 81 and the I-type layer 82 will have a step-like shape as shown in FIG. 7(B). This means a depletion layer 84 substantially reaches a junction surface 85 between the N-type layer 81 and the I-type layer 82.
However, there are some problems with regard to the PIN photodiodes 80 and 90 as following. As for the photodiode 90, although there is an advantage that the wavelength sensitivity of light rays depending on the thickness of the I-type layer 92 can be easily defined due to the easiness for controlling the thickness of the I-type layer 92, there is a problem that it can not deal with optical communication requiring a high-speed characteristic due to the shallowness of the depletion layer 94 which leads a response frequency characteristic F1 to be 10 MHz at most as shown in FIG. 6.
As for the photodiode 80, the depletion layer 84 substantially reaches the junction surface 85 of the N-type layer 81 and the I-type layer 82, and therefore about 200 MHz of a response frequency characteristic F2 can be obtained as shown in FIG. 9. However, it is substantially impossible to form a thick I-type layer with a thickness of, for example, at least 30 .mu.m (preferably at least 40 .mu.m) by the epitaxial growth, which causes the peak value of wavelength sensitivity to be shifted to a short wavelength side. This makes a problem that the peak value can not be conformed to the oscillating wavelength of a semiconductor laser which is practically used.
In addition, since the epitaxial growth process by which the I-type layer 82 in the photodiode 80 is formed is a difficult process, it takes too much time for forming the I-type layer 82 even if the thickness of the I-type layer 82 is restricted to, for example, 20 .mu.m, and leads the PIN photodiode 80 to a low yield, which in turn raises the cost of the PIN diode 80.